From the original invention of the wave gear device by C. W. Musser (U.S. Pat. No. 2,906,143) up to the present, a variety of inventions have been proposed by Musser and numerous other researchers, including the present inventor. A variety of inventions have been proposed in regard to the tooth profile alone. One such invention proposed by the present inventor is a method for designing a tooth profile that applies the technique of rack approximation to the meshing between teeth of a rigid internally toothed gear and teeth of a flexible externally toothed gear, whereby addendum profiles of both gears enabling wide-range tooth engaging therebetween is derived (JP 45-41171 B). An application has also been filed for an invention used to avoid tooth profile interference generated by rack approximation (JP 7-167228 A).
There is known a flat wave gear device configured to have an annular flexible externally toothed gear disposed within two rigid internally toothed gears arranged in parallel, and an elliptical wave generator mounted in the interior thereof (JP 2503027). A typical flat wave gear device is shown in FIG. 6. In a flat wave gear device 100, one rigid internally toothed gear 102 has the same number of teeth as a flexible externally toothed gear 104, and another rigid internally toothed gear 103 has 2n more teeth (n is a positive integer) than the flexible externally toothed gear 104. In the present specification, the rigid internally toothed gear having a different number of teeth than the flexible externally toothed gear is referred to as the “S-side rigid internally toothed gear,” and the rigid internally toothed gear having the same number of teeth as the flexible externally toothed gear is referred to as the “D-side rigid internally toothed gear.”
When a wave generator 105 having an elliptical contour is caused to rotate, counter-rotation occurs between the flexible externally toothed gear 104 and the S-side rigid internally toothed gear 103, which have different numbers of teeth. For example, by securing the S-side rigid internally toothed gear 103 so as to prevent rotation, and supporting the D-side rigid internally toothed gear on the other side in a rotatable state, reduced-speed rotation will be outputted from the D-side rigid internally toothed gear 102.
In order to prevent an increase in flexural stress caused by elliptical deformation of the flexible externally toothed gear at low reduction ratios (e.g., 60 or higher) in a flat wave gear device, the degree of radial deflection κmn (κ being the flexing coefficient, and m being the module of both gears) must be reduced to κmn (κ<1) from mn (κ=1), which is the normal degree of deflection (value obtained by dividing the pitch diameter of the flexible externally toothed gear by the reduction ratio when the rigid internally toothed gear is fixed). Since tooth depth is related to the degree of deflection, reducing the deflection leads to a decrease in the tooth depth, and in turn to a decrease in the ratcheting torque.
In order to prevent ratcheting under high load torque, it is necessary to increase tooth depth as much as possible, and the meshing region must be maximally enlarged in association therewith. However, tooth profiles that prevent ratcheting, which is a phenomenon whereby tooth-jumping occurs under high load torque, have yet to be proposed for flat wave gear devices having a low reduction ratio of 60 or less, such that the problem is addressed while continuous contact is maintained at high degree.